1. Field of the Invention
The present invention relates to a circuit for discriminating the modulation type of a received signal. More particularly, the invention relates to a circuit for discriminating the modulation type of a received signal, capable of being used for digital radio communications such as urgent communications over an ordinary communications line by using a signal of a modulation type different from the ordinary communications.
2. Related Background Art
For example, the Okinawa meteorological satellite communications line ordinarily used by communications between remote islands is also used as an urgent communications line by switching between both the lines which use the same frequency band. In such a radio communications system, a different modulation type is used for the ordinary and urgent communications lines. In the case of the Okinawa communications line, QPSK (Quadrature Phase-Shift Keying) is used for the ordinary communications line and BPSK (Binary Phase-Shift Keying) is used for the urgent communications line. In order to receive a signal of the urgent communications, it is desired to correctly and quickly discriminate the received signal modulation type.
A radio wave (carrier) transmitted in such a radio communications system changes its frequency with some factors such as a temperature. In order to track this frequency change on the receiver side, an automatic frequency control (AFC) circuit is used. Generally in a reception operation of a signal having a desired frequency, the AFC circuit of the radio communications system first scans a desired frequency and then tracks a change in the scanned and locked frequency. The operation of scanning a desired frequency is called an RF scanning mode, and the operation of tracking a change in the scanned and locked frequency is called a lock mode.
For the communications of a plurality of digital communications systems using the same frequency on the radio communications line, burst mode transmission/reception is used in which a time axis is segmented into bursts and transmission/reception of each digital communications system is allowed only during a particular burst period assigned thereto. Immediately after this particular burst is received during the RF scanning mode, the operation is switched to the lock mode and the particular burst is maintained to be received. If a reception of the particular burst is terminated and a different burst is received, the operation is switched to the RF scanning mode.
It is therefore necessary to judge whether a received burst is a desired one. Receiving a desired burst means receiving a signal of a desired modulation type. Namely, it is necessary to discriminate the modulation type of a received signal in order to switch between the RF scanning mode and lock mode of burst mode transmission/reception.
In a conventional circuit such as shown in FIG. 8 for discriminating the modulation type of a received signal, a PLL 21 extracts data clocks from received and demodulated data, the demodulated data is decoded by a decoder 22 by using the extracted data clocks, a sync code is detected from the decoded data by a sync code detector 23 by using the extracted data clock, and the modulation type of the received signal is discriminated in accordance with the sync code.
This circuit for discriminating the modulation type of a received signal is, however, complicated and takes some time to decode the modulated data and detect the sync code, being unable to discriminate the modulation type at high speed.
Still further, with the conventional received signal modulation type discriminating circuit, although the preamble data and other data are positioned at the start of the data format, it is necessary to detect the sync code positioned after the preamble data. Therefore, there is a time delay between the reception of a radio wave and the discrimination whether the received radio wave is of the desired modulation type.
It takes, therefore, a long time to switch the RF scanning mode of the AFC circuit to the lock mode after the signal of the desired modulation type is received. A start portion of the desired burst is therefore lost. Still further, if the sync code is not detected during the reception of the desired burst because of noises and jitters, the discrimination results in that a signal of the desired modulation type is not received, although the signal of the desired modulation type is being actually received.